Document handlers and counters for such documents as currency, checks, food stamps or the like are well known in the art, being disclosed in such references as U.S. Pat. Nos. 4,474,365 and 4,741,526. In such systems, it is important to obtain an accurate measurement of the length of the sheets being processed to detect such abnormalities as over-lapped notes, "chain notes", half notes in the direction of feed or the like. Typically, in such systems, the sheets are advanced widthwise past a pair of transversely spaced sensors, and the sheet length (i.e., dimension in the direction of feed) is determined from the period of time required to traverse the sensors. To improve the accuracy of the length measurement, the duration is often determined by counting pulses that are generated by a shaft encoder synchronously with the rotation of the feed member.
Such systems work well enough if the sheets are traveling with their leading edges perpendicular to the direction of movement. Owing to inevitable variations in driving force, this almost never happens, and the sheets are skewed to some extent. Any attempt to measure the length of such a skewed sheet simply by measuring the duration that either sensor is actuated or the duration that both sensors are actuated will result in inaccuracies.
U.S. Pat. No. 4,741,526, referred to above, discloses a system which determines the degree of skew from the delay between the time the leading edge of the document is detected by the first of the transversely spaced sensors and the time the leading edge is detected by the second of the sensors. The system then calculates a document length by multiplying the apparent length between the leading and trailing edges of the document by the cosine of the skew angle. Systems of this type are subject to error, however, when a document has not completely entered a feed nip and is slipping relative to the drive train. In such an instance, a long sensor actuation will erroneously be interpreted as a long document.